1. Field of the Invention
The present invention relates to a wafer susceptor designed for disposing a semiconductor wafer which susceptor is provided in a reaction chamber of a semiconductor manufacturing apparatus or the like.
2. Description of the Prior Art
There are utilized various kinds of methods of disposing a wafer upwards, perpendicularly or downwards in time of being subjected to the treatments based on the following techniques or a device. These techniques or the device involve a film forming device for forming thin films composed of multiple materials on the basis of a CVD (Chemical Vapor Deposition) technique, a plasma CVD technique, a sputtering technique and a bias sputtering technique, and further involve a reactive ion etching (RIE) technique, an ECR (Electron Cyclotron Resonance) etching technique and an impurity adding technique such as an ion implantation technique or the like. The wafer is disposed on a wafer susceptor, this largely depending on a method of mechanically supporting the circumferential portion of the wafer. Hence, it is a quite common method that an operator sets the wafer at a predetermined position by opening the reaction chamber wherein the wafer undergoes various kinds of treatments, or the operator sets the wafer on a wafer holder in a pre-chamber prepared before the reaction chamber and this wafer holder is transferred to the reaction chamber with the aid of a load lock mechanism (a wafer carrying device) in order to set it at the predetermined position. In such a method, however, inasmuch as the reaction chamber itself or the wafer holder is considerably suffers atmospheric pollution, a clean-process is far from being actualized. The importance of clean process for realizing submicron patterning in the future has fully been disclosed in the specification, titled "Apparatus For Treating Wafer", of Japanese Patent Application No. 211643/1985 which was made by the present inventors. However, a description relative to this will herein be made once again.
Advancement of an LSI technology is now being accelerated with amazing rapidity. In such circumstances, there has already been manufactured an LSI which makes use of a minute pattern of less than 1 .mu.m. As a matter of fact it requires a manufacturing process invested with still more favorable controllability and high performance to produce the LSI of the above-mentioned submicron level, this process being less influenced by uncertain elements than other processes.
As one example, the lowering of temperature and a high selectivity process are exemplified. Chiefly, the lowering of temperature is needed for restraining re-diffusion of impurities within the semiconductor and for actualizing accurate impurity distribution. Especially, the lowering of temperature is effective both in restraining grain growth in various kinds of thin films (a semiconductor, metal and an insulating material) and in controlling interface reaction between substrate crystal and a thin film thereon and between the thin films. On the other hand, high selectivity among different materials is indispensable for etching or thin film deposition in fine patterning processes.
The most essential condition for fulfilling the lowering of temperature and the high selectivity process is that unnecesary gaseous components except the gaseous components required for the reaction are almost completely eliminated from the atmosphere in the reaction chamber where the process is proceeded. Videlicet, an ultra-clean process is the inevitable factor for actualizing the low-temperature process and the high selectivity process which are indispensable for manufacturing infinitesimally by small geometry LSIs.
The significance of the clean process will be described by exemplifying a few examples as follows:
(1) Silicon epitaxial growth: EQU SiH.sub.4 .fwdarw.Si+2H.sub.2 ( 1) EQU SiH.sub.2 Cl.sub.2 +H.sub.2 .fwdarw.Si+2HC+H.sub.2 ( 2)
The epitaxial growth of silicon is effected by thermal decomposition (formula (1)) of silane (SiH.sub.4) and by hydrogen reduction reaction (formula (2)) of dichlorosilane (SiH.sub.2 Cl.sub.2).
However, if oxygen (O.sub.2) or moisture content (H.sub.2 O) is present in the reactive atmosphere, the surface of a silicon substrate will be consecutively oxidized, thereby forming a thin film of SiO.sub.2 on the surface. A high temperature is needed for removing the thin film of SiO.sub.2 by evaporation or etching.
If the reactive atmosphere is extremely clean, the clean silicon surface can invariably be obtained. Surface migration of Si atoms adsorbed on the clean surface takes place intensively, thereby readily putting the atoms adsorbed on the surface into a normal lattice site and further obtaining the epitaxially grown layers with high quality and low temperature. Namely, it is feasible to decrease the process temperature in the clean atmosphere
(2) Tungsten selective growth: EQU 2WF.sub.6 +3Si.fwdarw.2W+3SiF.sub.4 ( 3)
Tungsten (W) is selectively grown on the single-crystalline silicon (Si) or the polycrystalline silicon (Si) by employing a raw gas WF.sub.6, this selective growth being progressed by Si substrate reduction reaction.
An oxide film or the like must not exist in an Si-W interface in order that the reaction expressed by the formula (3) is progressed; and as a matter of course, W itself must not contain the oxide film or the like. Si and W are highly oxidizable materials.
Consequently, as long as the reactive atmosphere contains O.sub.2 and H.sub.2 O, the substrate reduction reaction shown in formula (3) never occurs. At the interface between Si and the metal which does not interpose an intermediate layer such as the oxide film or the like therebetween, Si-Si bonding is disconnected at an exremely low temperature by the shielding effect of the metal, whereby the Si atoms diffuse over the metal thin film surface along grain boundaries of polycrystalline metal. Subsequently, the substrate reduction reaction expressed by the formula (3) continues to occur.
It is quite natural that W should not be accumulated on the surfaces of, for instance, SiO.sub.2 and Si.sub.3 N.sub.4 except Si, as long as the substrate reduction reaction is adopted. That is, provided that the reactive atmosphere is clean, W is selectively acculumated on Si alone at a low temperature. As explained earlier, that the reactive atmosphere is clean makes both the lowering of process temperature and the high selctivity process possible.
For the purpose of making the reactive atmosphere clean, cleaness is strictly indispensable for all the systems such as a gas supply system ranging from a raw gas cylinder (or liquefied gas vessel hereinafter referred to as a gas cylinder) to the reaction chamber, the reaction chamber itself and a gas pumping system.
The principal conditions are enumerated as follows:
(1) The raw gas has as high purity as possible.
(2) In connection with the gas supply system, the reaction chamber and the discharging system,
(a) an amount of outside leakage is decreased to its minimum, and no atmospheric pollution is present.
(b) gases released from a piping system and the tube wall of the reaction chamber are considerably low in quantity. Namely, the materials of which the piping system and the reaction chamber are composed do not contain any gaseous components. Simultaneously, the surface of the tube wall is sufficiently flat and includes no processed alterative layer, and adsorbed gas is considerably low in amount. Furthermore, there is equipped a device in which the inside of the reaction chamber and the inside of the gas supply piping system are so arraged as not to be exposed to the atmosphere.
(c) there exists no dead zone in which the gas stays.
(d) the number of particles which are to be produced is rendered as small as possible. Even if a movable mechanism is provided, sliding portions are, for preference, not formed in the gas supply system and the reaction chamber.
Some of the above-described necessary conditions can be fulfilled by management or operations. However, the occurrence of the particles in the movable system of the device is the very problem in constructing the mechanical system itself in the wafer carrying device, and its structure is important.
In this case, the wafer carrying mechanism which transfers the wafer to the reaction chamber is of particular importance. This wafer carrying mechanim requires a mechanism whereby the wafer is supported or held.
To cope with the aforementioned defects, there is developed a technology in which the wafer alone is transferred to the reaction chamber, and the wafer is adsorbed on the susceptor by dint of an electrostatic force. For example, the Reactive Ion Etching Device Hirie-100 made by Tokuda Mfg. Co., Ltd. has already been put into practical use. However, inasmuch as the structure is such that the overall surface of metal such as stainless-steel or the like is covered with an insulating material, the above-described etching device is attended with defects wherein the electric potential control of the adsorbed wafer loses certainty, and separation of the wafer from the susceptor is contingent on a mechanical means. Moreover, in the specification of the aforecited Japanese Patent No. 211643/1985 is disclosed an apparatus for carrying the wafer by electrically adsorbing it with the help of an electrostatic chuck provided between the reaction chamber and the pre-chamber.